You can get a dramatic reduction in price without sacrificing comfort and safety by selecting a Paris suburb that has good Metro connections...
Saint-Cloud is a short walk to 'Boulogne-Pont de Saint-Cloud' metro station and a quick search locates decent hotels starting at about EUR 85.
La Défense has its own metro station and a quick search locates brand name hotels starting at about EUR 100 (better deals on the weekends)
Sèvres has brand name hotels in the same price range as Saint-Cloud, but requires a 15 minute walk across the bridge to reach the metro station.
The commute from these places to central Paris ranges from 30 (La Défense) to 60 minutes (Sèvres).
These places are in the western suburbs of Paris. I find that area more appealing for reasons vaguely related to safety and long-standing familiarity (generally not worth going into here), but there would be similar opportunities in the southern suburbs also.
The answer to your question: "Why are hotels in Paris too expensive?" is because they can.